This invention relates to a liquid developer for development of electrostatic latent images such as in electrophotography or electrostatic recording and more particularly, it relates to a liquid developer which is excellent in preservation property, stability and fixability and especially can provide images of high quality.
There have been known a number of liquid developers for development of electrostatic latent images. For example, the well known method comprises mechanically dispersing colorants, pigments or dyes such as carbon black, cyanine blue, nigrosine, oil dyes, etc. in a highly insulating medium together with resins such as rosin, alkyd resins, acrylic resins, synthetic rubbers, etc. by a ball mill, an attritor, a homogenizer, etc. and further adding a metallic soap, an amine, a higher fatty acid, etc. to stably give electric charges to the dispersed particles of the dispersion.
However, the liquid developer obtained by the above method has the defect that since the dispersed particles as a developer have a broad distribution of their diameter, a lot of precipitates are produced, it is inferior in charge stability and production stability and stable images are obtained with difficulty.
Japanese Patent Examined Publication Nos. 54029/78 and 12985/82 disclose a method which comprises previously introducing a polymerizable vinyl group into a precursor polymer, polymerizing a monomer in the presence of said precursor polymer to produce a graft copolymer and coloring this graft copolymer with a dye. This method, although being excellent, still has problems in control of introduction of vinyl group and reproducibility and often causes formation of gel or sometimes it is difficult to stably produce dispersed particles resulting in formation of coarse particles of more than 1.mu. or fine particles of less than 0.1.mu.. These are not preferred in practical use.
Furthermore, Japanese Patent Unexamined Publication Nos. 83174/84, 177572/84, 212850/84, 212851/84, 164757/85, 179751/85, 185962/85, 185963/85, 252367/85, 116364/86, 116365/86, etc. disclose use of resin dispersion as a liquid developer which is produced by polymerizing a monomer which is soluble in a highly insulating medium in the presence of a polymer soluble in said medium, but becomes insoluble therein upon being polymerized. This method is excellent in stability, in industrial production an in dispersion stability. However, when images are formed on an electrostatic recording material by coulombic force using a liquid developer prepared from the resin obtained by this method, there occur electrostatic mutual repulsion due to coulombic force of the dispersed particles per se and excluded volume effect caused by repulsion due to increase in concentration of dispersed and stabilized polymer. Therefore, there is the defect that with decrease in effective surface potential of electrostatic recording material, blurred ghosts are apt to occur around images. There is another defect that when the liquid developer prepared from the resins obtained by the above method is continuously used with replenishment of the developer, solvent soluble resin is accumulated to affect the charge quantity of dispersed particles, although it depends on the kind of the soluble resin.
Moreover, in order to use the developer for preparation of printing plate where toner images as referred to hereinafter is functioned as a resist layer, the toner particles are preferably internally crosslinked because a resist layer formed of particles having crosslinked structure by fixation and film-formation has a high resisting ability and can provide goo printing plates. None of the conventional wet developers satisfy all of these characteristics.